The Secret of Life on Earth

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Every form of life on Earth

depends on the ability of plants...

...to capture the sun's energy

and transform it into food.

Sea birds feed on fish,

and bigger fish eat smaller fish.

But somewhere their food chain

began with plants.

Every food chain is started

by a vegetarian.

The great African herds

of wildebeest feed on grass.

And in their turn, they become food

for predators such as lions.

lt is plants, and plants alone, which

generate all the food we have on Earth.

Without them,

ours would be a dead planet.

And four billion years ago,

that's just what it was...

...dead.

A forbidding, hostile place...

...bombarded by ultraviolet radiation...

...the planet's surface

sealed with poisonous gasses.

Yet in such conditions,

strange carbon compounds formed...

...with power to reproduce themselves.

From them appeared the first

primitive cells of life.

Drawing energy from the sun,

they began to make their own food.

Some combining water with a chemical

we know as chlorophyll...

...and this produced an important

byproduct:
bubbles of oxygen.

An atmosphere was being created

in which...

...you and l would eventually be able

to breathe and live.

Two billion years ago, this was

the scene where land and sea met.

ln the next billion years, the oxygen

was gradually accumulating...

...some of it forming a shield of ozone

in the upper layers of the atmosphere.

At last, the stage was set

for life in many forms.

And true plants swarmed

in prehistoric waters.

Now, filter-feeding animals

such as barnacles...

...could flourish

in the oxygenated water.

The secret of life was formed

in an unwritten contract...

...binding plants and animals

together:
the green contract.

Some animals might live by

eating other animals...

...but every food chain must start

with animals eating plants.

That is the essence

of the green contract.

By the time fish had evolved...

...another stage in the story

of life was beginning.

The teeming life of the oceans

reached out to the dead land.

About 400 million years ago,

the first plants came ashore.

Tiny growths

clinging to rocks and mud.

ln time,

they took a firmer hold on land.

They developed roots.

Plants developed woody vessels within

their stems which made them rigid...

...and enabled them to grow tall.

Competition with other plants

compelled them to grow taller still.

With this dynamic lift off,

the first forests were born.

Plants had successfully

invaded the land.

Their roots broke the Earth's surface

and held together the topsoil.

The atmosphere they'd created made life

on land possible for many animals...

...among them, insects.

Wings meant that insects could follow

the plants upwards into the sky.

The first land plants

had swimming sex cells...

...and could therefore

thrive only in moist places.

But by degrees, plants developed

new methods of reproduction.

The conifers' male sex cells could be

carried on the wind to female cones...

...which produced seeds.

Since they reproduced in this way...

...the trees could move into new,

dryer territories.

Like an advancing army,

they swarmed the foothills...

...and occupied the mountainsides.

A far cry from the shoreline

of the ocean.

Today, one-third of all

the world's forests are conifers.

Among them, the largest living

organisms:
the giant redwoods...

...capable of growing to the height

of a 20-story building.

A remarkable development...

...but there was an even more

spectacular one still to come.

The plant's male and female parts were

combined in a single marvelous device:

The flower.

Flower pollen is an attractive source

of food for insects.

They carry some of it on their bodies

as they move from flower to flower.

ln that way,

the flower's eggs may be fertilized.

Many flowers produce nectar,

which attracts insects...

...with guidelines

to help them find it.

Some flowers

favor particular insects.

The wild arum has a spike

with a fetid odor...

...which attracts small mosquitoes.

Many slide to the bottom

where the female parts are located.

They remain trapped overnight.

But next day,

the male parts exude pollen.

The bristles wilt and clear the way

for the mosquitoes to escape...

...well-coated with a dusting of pollen

that they carry to the next arum.

Again, temporary prisoners...

...they pollinate their captor

before escaping once more.

The size and shape of certain flowers

match the bills of particular birds.

Members of the parrot family,

like the rainbow lorikeet...

...have adapted

to eat certain flowers...

...and so

have become their chief pollinator.

Other flowers await pollination

at night.

ln Australia, small nocturnal

marsupials called honey possums...

...visit banksia flowers

for the sake of their nectar.

By chance, they carry pollen

on their fur...

...as they move from flower to flower.

The traffic to the flowers

that feed them...

...is an open invitation

to the secret hunters.

Some predatory mantes can disguise

themselves as flower petals...

...to fool their victims.

The killer, poised to strike.

Life in the world

of flowers became complicated...

...with many variations on the theme

of ''predator meets prey.''

ln the relationships

linking plant and animal...

...it was not always the plant

which was the passive partner.

ln some cases, the plants themselves

became the killers.

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